The invention relates to the design of fractionation trays and columns used to separate volatile chemical compounds by fractional distillation. The invention specifically relates to the design and construction of the downcomers of cross flow trays of a dividing wall fractionation column.
Fractional distillation is used to perform a large number of separations of volatile chemical compounds in the chemical, petrochemical and petroleum refining industries. Large scale commercial fractionational distillation is conducted in a vertical enclosed vessel referred to as a fractionation column.
The fractionation column will contain some form of vapor-liquid contacting device which may be in the form of packing such as random or structured packing or in the form of a fractionation tray. A fractionation tray typically comprises a large flat area referred to as the decking or contacting deck of the tray plus means to deliver liquid to the tray from the next tray above and to remove liquid for passage to the next tray below. The liquid being removed from the tray flows through a part of the tray referred to as a downcomer. Vapor generated in the lower portion of the column passes upward through perforations in the decking, while the liquid flows downward from tray to tray countercurrent to the vapor. With a cross flow type tray the liquid first enters upon a tray from the downcomer of the tray above. It then passes across the decking of the tray and finally exits through an outlet downcomer of the tray.
The material actually entering the outlet downcomer is normally in the form of a froth or a liquid containing entrained bubbles or some other form of two-phase admixture of vapor and liquid. One function of the downcomer structure is to promote the separation of entrained vapor from the liquid such that only liquid passes downward to the next tray. This is necessary for optimum performance of the column.
U.S. Pat. No. 2,471,134 issued to R. O. Wright illustrates a dividing wall fractionation column having a partition or dividing wall 20 dividing the trayed column into two parallel vapor-liquid contacting chambers. A similar but more detailed disclosure of a dividing wall fractionation column is provided by U.S. Pat. No. 4,230,533 issued to V. A. Giroux. Dividing wall columns are closely related to a different type of column referred to as a partitioned distillation column such as illustrated in U.S. Pat. No. 5,755,933 issued to Thomas P. Ognisty et al. A partitioned distillation column differs from a dividing wall column in that the vertical dividing wall is positioned such that it contacts one end of the column. Thus only one terminal portion of the column is divided into the two parallel contacting sections. In this manner two overhead products or two bottom products may be removed from a single column. A dividing wall column produces an intermediate boiling fraction.
Despite the fact that the two parallel contacting chambers in the central portion of a dividing wall column are in open communication at their upper and lower ends, the conditions in these two chambers can differ. Specifically, the temperature in the two chambers may differ at the same horizontal height in the column. As the dividing wall and essentially all of the fractionation column will typically be made of metal, there exists the possibility for heat transfer from one chamber to the other. This has led to the development of dividing wall columns which have an insulated dividing wall as illustrated in U.S. Pat. No. 5,785,819 to G. Kaibel et al. This reference discloses the use of actual insulation, a vacuum or inert gases to insulate the dividing wall.
The effect of heat transfer across the wall of dividing wall columns was investigated in a paper presented by F. Lestak et al., published at page 639-644, of TranslChemE, Vol. 72, Part A, September 1994. This paper indicates that horizontal heat transfer through the dividing wall may be beneficial or detrimental to the separation which is performed in the column. It suggests that insulation can be applied to the portion of the dividing wall where the heat transfer is detrimental. Where heat transfer is beneficial, it can be promoted or enhanced as by the use of fins, heat pipes, etc.
In prior art dividing wall column cross flow trays the dividing wall has been used as the back wall of the downcomers located near the center of the column. The invention is a new design for the downcomers of cross flow fractionation trays used in the divided section of a dividing wall column. The novel design of the subject invention mitigates localized heat transfer through the dividing wall which can vaporize some of the liquid present in the downcomer of the tray. The formation of vapor in the downcomer interferes with fluid flow through the downcomer and the separation of vapor and liquid within the downcomer. The subject invention adds a parallel isolation wall which separates the downcomer from the dividing wall thus providing thermal insulation and reducing vaporization in the downcomer. It eliminates a need to insulate the entire dividing wall.
A broad embodiment of the invention may be characterized as a cross flow tray for performing fractional distillation in a dividing wall fractionation column, the column comprising a central dividing wall and having a curved inner surface, which tray comprises perforated decking occupying at least a central portion of the tray and defining the contacting level of the tray; a liquid receiving area located on a first side of the tray, the first side of the tray being curved to generally conform to the inner surface of the column; an outlet weir projecting upward from the contacting level of the tray along a chord across the tray to define an inlet of a downcomer located on the second side of the tray opposite from the liquid receiving area, and an isolation wall located between the weir and the dividing wall, the isolation wall being substantially parallel to the weir and extending downward from the contacting level of the tray to the outlet of the downcomer, the wall being offset from a substantially linear edge of the tray, which linear edge generally conforms to the dividing wall, to define an isolation volume located between the dividing wall and the isolation wall.